The present invention relates to semiconductor devices having a protection device for protecting the internal circuit against electrostatic discharge and other abnormal voltage.
A semiconductor device is provided with a protection device to protect its internal circuit against electrostatic discharge and other abnormal voltage. An example of the protection device is described in “Coupled Bipolar Transistors as Very Robust ESD Protection Devices for Automotive Applications” (2003 EOS/ESD Symposium). This device uses a thyristor which includes a vertical bipolar transistor and a horizontal bipolar transistor.
For the protection device, an important parameter is trigger voltage. The trigger voltage must be higher than the supply voltage and at the same time it must be lower than the withstand voltage of the device to be protected, or protected device. Therefore, it is desirable that the trigger voltage of the protection device can be freely decreased depending on the withstand voltage of the protected device.
For example, Japanese Patent Publication No. 2008-172201 describes a technique that a protection device uses a thyristor including a vertical bipolar transistor and a horizontal bipolar transistor and the base of the horizontal bipolar transistor has a double layer structure comprised of a lower doped layer and a highly-doped layer. According to this technique, the trigger voltage can be controlled by adjusting the distance between the two layers.
U.S. Pat. No. 7,667,295 describes that a protection device with a vertical bipolar transistor has a lower doped base region around the base region.